Factors that affect gene expression
There are a number of factors that regulate the phenotypic expression of genes.
Pre-transcription impacts
At transcription, RNA can only attach to a spot called the promoter. In eukaryotic cells, this happens with the support of proteins called general transcription factors. These are proteins that are produced by regulatory genes that help to control gene expression.
However, other transcription factors can block the attachment of RNA polymerase, meaning to gene is expressed, or even alter the rate at which a gene is expressed.
Example of pre-transcription impacts: The Lac Operon
The lac operon of E. coli consists of genes that are involved in the metabolism of lactose. The genes code for proteins that allow E. coli to utilise lactose as a source of energy. However, it is only expressed when lactose is present and glucose is absent.
The lac repressor acts as a lactose sensor. It normally blocks transcription, but is released and therefore stops acting as a repressor when lactose is present.